In cutting inserts (hereinafter referred to as “inserts” in some cases), it has conventionally been proposed to form a cutting edge in a projected and concave shape by disposing a groove part in a side face, from the viewpoint of reducing chip twining (for example, refer to Japanese Unexamined Utility Model Publication No. 2-53305).
However, the groove part in the conventional insert is formed to extend from the upper surface to the lower surface of the side surface. Therefore, the wall thickness of the insert, namely, the distance from a cross section, which includes the central axis of the insert and is parallel to the cutting edge, to the side face cannot be ensured continuously in the width direction of the side face, and hence there is a risk that the fracture resistance of the insert will deteriorate. In other words, by the presence of the groove part, the portion adjacent to the groove part is projected in a block shape extended from the upper surface to the lower surface of the side surface. There is also a risk that when cutting is performed by the cutting edge located at the end portion on the upper surface, the portion projected in the block shape will not withstand cutting stress, causing a fracture.
Hence, there is a need for an insert having low cutting resistance and excellent fracture resistance. Particularly, in the insert that employs a through hole (attachment hole) extending from the upper surface to the lower surface, the through hole contributes to a further decrease in the wall thickness of the insert. This requires further enhanced measures against the fracture resistance deterioration.